If you have forgotten your mailbox password, you will just need to change it. To change your mailbox password, you first need to log into the Purple Serve Control Panel for the domain which the mailbox was created on. You can then manage the mailboxes for the domain and select the mailbox which needs to be changed.
If you do not have access to a Purple Serve Control Panel for your domain please raise a support ticket or send an email to support@purpleserve.net detailing the email account which you cannot access.